The present invention relates to a method to generate magnetic fields of high uniformity within the object examined and compensation of external dispersed field, and a system for its embodiment, which method is applicable in magnetic resonance tomography.
The simplest way to generate a uniform magnetic field is with the use of a Helmholtz coil system. However, a uniform field is obtained only in a small region, between the coils. For instance, uniformity of the magnetic field [generated] with the use of such coils having a radius R=1 m will be of the order of ±1000 ppm for a region between the coils −0.2 m≦z≦0.2 m. Such uniformity is too low for applications in nuclear magnetic resonance (NMR) tomography or in electron paramagnetic resonance (EPR) tomography. The uniformity of magnetic field which is required in either of the two examination techniques must be in the order of several dozen ppm or lower. A possible way to improve the uniformity of magnetic field within the object examined is by using two pairs of coils having different radiuses and different current flow directions (George A. Rinard et al. Magnet and Gradient Coil System for Low-Field EPR Imaging, Conc. Magn. Res. 15 (2002), 51-58). The uniformity of the magnetic field obtained in this case for two pairs of cells having radiuses R1=40.64 cm and R2=19.77 cm, within −0.075 m≦z≦0.075 m was in the order of ±40 ppm. This is a considerable progress, compared with Helmholtz's single coils having a radius of R=40.64 cm, for which uniformity is in the order of ±600 ppm. The above solution enables magnetic fields to be generated with sufficient uniformities, however, a smaller diameter of the second coil narrows the size of the object to be examined which, for the size of human body entails the construction of a large, energy consuming system. Therefore, in the Center for EPR Imaging, Chicago (Epel B, Sundramoorthy S V, Mailer C, Halpern H J, A versatile high speed 250-MHz pulse imager for biomedical applications, Concepts in Magnetic Resonance Part B: Magnetic Resonance Engineering. 33B(3):163-176, 2008) a system was built for the examination of small organisms which was resealed twice, compared with the parameters presented in the paper referred to above. In addition, opposite current flow directions in both coil pairs decrease the magnetic field intensity, which involves the use of more power.
Such limitations are not observed in the layout of the coil systems in an electromagnet, which enables very high uniformities of the magnetic field to be obtained within the working space of a magnetic resonance tomograph.